Synthesis, spectra and crystal structure of
(E)-(CO)
2(NO)Cr[(
h
5-C
5
H
4) – CH
CH(h
5-C
5H
4)]Cr(CO)
2(NO)
Yu-Pin Wang
a,*, Xen-Hum Lui
a, Bi-Son Lin
a, Wei-Der Tang
a, Tso-Shen Lin
a,
Jen-Hai Liaw
a, Yu Wang
b, Yi-Hung Liu
baDepartment of Chemistry, Tunghai Uni6ersity, Taichung, Taiwan, ROC bDepartment of Chemistry, National Taiwan Uni6ersity, Taipei, Taiwan, ROC
Received 14 April 1998; received in revised form 19 September 1998
Abstract
Compounds 1,2-bis[(h5-cyclopentadienyl)dicarbonylnitrosylchromium]ethene (7) (hereafter called 1,2-dicynichrodenylethene)
and 1,2-diferrocenylethene (10) were prepared from formylcynichrodene (3) and formylferrocene (9), respectively, via the McMurry’s low-valent titanium coupling method. Compounds (h5-vinylcyclopentadienyl)dicarbonylnitrosylchromium (6) and
vinylferrocene (11) were obtained by the dehydration of the corresponding alcohols. The structure of 7 was solved by an X-ray diffraction study: space group, P21/c; monoclinic; a = 6.379(5), b = 11.295(3) and c = 11.9352(24); Z = 2. It turns out that
compound 7 adopts a transoid conformation at the ethenylene bridge and the two cyclopentadienyl rings are coplanar. The nitrosyl group in each cynichrodenyl moiety of 7 is located at the side towards the corresponding ethenylene carbon atom with a twist angle of 46.5°. The chemical shifts of H(2) – H(5) protons and C(2) – C(5) carbon atoms of a series of vinyl derivatives of compounds bearing cyclopentadienyl rings have been assigned using two-dimensional HetCOR-NMR spectroscopy. For the derivatives of cynichrodene (1) and ferrocene, it was found that the shielding of C(2,5) and C(3,4) carbon atoms is parallel to the shielding of the ortho- and para-carbon atoms of benzene derivatives. The electron density distribution in the cyclopentadienyl ring is discussed on the basis of 13C-NMR data. Surprisingly, the vinyl group donates electron density to the adjacent
cynichrodene moieties rather than withdraws from them. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Chromium; Synthesis; Spectra; Crystal structure
1. Introduction
Earlier [1,2], we reported the unequivocal assign-ments of C(2,5) and C(3,4) on the Cp ring of cyn-ichrodene (1) derivatives bearing electron-donating or electron-withdrawing substituent in 13C-NMR spectra.
For derivatives with electron-donating substituents, an analogy was observed between the shielding of C(2,5) and C(3,4) carbon atoms of cynichrodene 1 deriva-tives and ferrocene derivaderiva-tives and that of the
ortho-and para-carbon atoms of benzene derivatives. For derivatives bearing electron-withdrawing substituents, the opposite correlation on the assignments was ob-served between cynichrodene derivatives and the derivatives of ferrocene and benzene. The high-field and low-field chemical shifts are assigned to C(3,4) and C(2,5), respectively, in the case of cynichrodene derivatives bearing electron-withdrawing groups, while the opposite assignment was made for the ferrocene derivatives. This amazing finding prompted us to study 6 – 7 and 10 – 11, compounds containing vinyl moiety, a group capable of being either electron-do-nating or electron-withdrawing.
Several methods have been employed to couple the
* Corresponding author. Fax: + 886-4-359-96851.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 3 2 8 X ( 9 8 ) 0 1 0 1 0 - 9
carbonyl groups to yield orefins [3,4]. However, the applications to organometallic compounds are limited [5]. Herein, we report the preparations of 7 and 10 via the McMurry’s low valent titanium method and the crystal structure of (E)-(CO)2(NO)Cr(Cp – CHCHCp)Cr(NO)(CO)2 (7). Spectral comparison be-tween benzene, ferrocene and cynichrodene derivatives bearing corresponding substituents is also included.
2. Experimental details
All the syntheses were carried out under nitrogen by the use of Schlenk techniques. Traces of oxygen in the nitrogen were removed with BASF catalyst and the deoxygenated nitrogen was dried over molecular sieves (3 A˚ ) and P2O5. Hexane, pentane, benzene, and dichloromethane were dried over calcium hydride and freshly distilled under nitrogen. Diethyl ether was dried over sodium and re-distilled under nitrogen from sodium-benzophenone ketyl. All the other sol-vents were used as commercially obtained.
Column chromatography was carried out under ni-trogen with Merck Kiesel-gel 60. The silica gel was heated with a heat gun during mixing in a rotary evaporator attached to a vacuum pump for 2 h to remove water and oxygen. The silica gel was then stored under nitrogen until use. Compounds 3 and 9 were prepared according to the literature procedures [6,7].
1H- and 13C-NMR were acquired on a Varian
Unity-300 spectrometer. Chemical shifts were refer-enced to tetramethylsilane. IR spectra were recorded with a Perkin-Elmer Fourier transform IR 1725X spectrophotomer. Microanalyses were carried out by the Microanalytic Laboratory of the National Chung Hsing University.
2.1. Preparation of(h5-6inyleyclopentadienyl)
dicarbonylnitrosylchromium (6)
This compound was prepared according to Mintz and Rausch [8] with minor modifications. [h5
-(1-Hy- droxyethyl)cyclopentadienyl]dicarbonylnitrosy-l-chrom-ium (8) (0.54 g, 2.2 mmol), 5 mg of hydroquinone, and 0.05 g of p-toluenesulfonic acid were dissolved in 50 ml of benzene. The mixture was refluxed for 1.25 h, then 100 ml of dichloromethane was added. The solution was washed with distilled water and then dried with anhydrous magnesium sulfate. After the solution was filtered and concentrated, 0.42g of a residue of 6 (84%) was obtained. An analytical sam-ple was prepared from centrafugal TLC (hex-ane:benzene (1:1)).
Proton-NMR (CDCl3): d (relative intensity,
multi-plicity, assignment): 5.01 (2H, t, Cp H(3,4)); 5.15 (1H, dd, HB, JAB= 3.3, JBX= 11); 5.20 (2H, t, Cp H(2,5)); 5.44 (1H, dd, HA, JAX= 17.7); 6.22 (1H, dd, Hx). 13C-NMR (CDCl3): d (assignment): 88.13 (Cp, C(2,5)); 89.74 (Cp, C(3,4)), 108.31 (Cp, C(1)); 115.13 (CH2); 128.70 ( – CH); 236.97 (Cr–CO). IR(KBr): n (cm− 1) (intensity): 2020 (vs), 1946 (vs), 1696 (vs), 628 (m). 2.2. Preparation of (E)-1,2-dicynichrodenylethene (7) Titanium tetrachloride (3.30 ml, 30 mmol) was added into a Schlenk tube containing lithium
alu-Table 1
Summary of crystal data and intensity collection of 7a
Empirical formula C16H10O6N2Cr2
Formula weight 430.25
Diffractometer used Nonius
Space group P21/c; monoclinic
a (A˚ ) 6.379(5) b (A˚ ) 11.295(3) c (A˚ ) 11.9352(24) 99.17(4) b (°) Volume (A˚3) 849.0(7) Z 2 Density (calculated) (g cm−3) 1.683 l (A˚) 0.7093 F(000) 432
Number of reflections for indexing 25 (14.44° 2u 26.00°)
Scan type 0–2u
Scan width (°) 2(0.70+0.35 tan(u)) Scan speed (° min−1) 2.06–8.24
2u maximum (°) 50 h, k, l ranges (−7, 7), (0, 13), (0, 14) 12.833 m (cm−1) Crystal size (mm) 0.05×0.20×0.50 Transmission 0.865, 1.000 Temperature (K) 298
Number of measured reflections 1492 1071 Number of observed reflections
(I\2.0s(I))
Number of unique reflections 1492 0.035, 0.027
Rf, Rw
Goodness-of-fit 1.86
Refinement program NRCVAX
Number of atoms 18
138 (1071 out of 1492 Number of parameters refined
reflections) Minimize function Sum(w Fo−Fc 2)
(1/s(Fo))2
Weights scheme
(D/s) max 0.0085
(D-map) max, min (e A˚−3) −0.260, 0.380
aR
f= Sum(Fo−Fc)/Sum(Fo); Rw= Sqrt[Sum(w(Fo−Fc)2)/Sum(wFo2)]; goodness-of-fit = Sqrt[Sum(w(Fo−Fc)
2/(no. of reflections−no. of parame-ters)].
Table 2
The contracted 2D HetCOR spectra of 2–7
2D HetCORb 13C, Cp(Cr)a Compound R 1H, Cp(Cr)a 2 COOH CHO 3 C(O)CH3 4 5 NH2 CHCH2 6 7 CHCH–(C6H5)Cr(CO)2(NO)
a, (2,5); , (3,4); the magnetic field increases towards the right.
bThe magnetic fields of1H- and13C-NMR spectra increase towards the right and upper side respectively.
minium hydride (0.57 g, 15 mmol) at 0°C. The temper-ature was slowly raised to 90°C and held for 12 h with stirring. After cooling down to 0°C, 100ml of tetrahy-drofuran and formylcynichrodene (3) (0.92 g, 4.0 mmol) were added into the residue. The reaction mix-ture was heated to 55°C and held for 16 h before cooling to 0°C, then 50 ml of a saturated potassium carbonate aqueous solution was added in slowly. Thereupon the reaction mixture was extracted with three 50 ml portions of dichloromethane. The combined extracts were washed three times with distilled water and dried with anhydrous magnesium sulfate. The solu-tion was filtered and concentrated to a residue. The residue was dissolved in 30 ml of dichloromethane. Three grams of silica gel were added to the solution, and the solvent was then removed under vacuum. The residue was added to a dry-packed column (2.0 × 30 cm) of silica gel. Elution of the column with hex-ane:benzene (3:1) gave two bands. The first band was unidentified. After removal of solvent, the second (red) band gave (E)-1,2-dicynichrodenylethene (7) (0.31 g, 36%). An analytical sample (m.p. 186.5°C) was pre-pared by recrystallization using the solvent evaporation method from pentane:dichloromethane (5:1) at 0°C.
Anal. Found: C, 44.53; H, 2.46; N, 6.66.
C16H10Cr2N2O6 Calc.: C, 44.67; H, 2.34; N, 6.51%.
Proton-NMR (CDCl3): d (relative intensity,
multiplic-ity, assignment): 5.05 (2H, t, Cp H(3,4)); 5.25 (2H, t, Cp H(2,5)); 6.31 (2H, s, – CH). 13C-NMR (CDCl 3):d (assignment): 88.57 (Cp, C(2,5)); 89.93 (Cp, C(3,4)), 106.72 (Cp, C(1)); 121.59 ( – CH); 236.70 (Cr–CO). IR(KBr):n (cm− 1) (intensity): 2010 (vs), 1932 (vs), 1682 (vs), 644 (m). Mass spectrum: m/z: 430 (M+). 2.3. Preparation of (E)-1,2-diferrocenylethene(10) To a solution of titanium trichloride (2.80 g, 18 mmol) in tetrahydrofuran, lithium aluminium hydride (0.34 g, 9.0 mmol) was added in an ice salt bath. After stirring for 30 min, formylferrocene (9) (1.00 g, 4.6 mmol) in 20 ml of tetrahydrofuran was added into the deep black solution. After stirring for 3 h at room temperature, 20 ml of hydrochloric acid (4 N) was added. The reaction mixture was then extracted with three 50 ml portions of dichloromethane. The combined extracts were washed three times with distilled water and dried with anhydrous magnesium sulfate. The solu-tion was filtered. Ten grams of silica gel were added and the solvent was then removed under vacuum. The residue was added to a dry-packed column (2.0 × 10 cm) of silica. Elution of the column with hexane/ben-zene (1:1) gave two bands. The first band was uniden-tified. After removal of solvent under vacuum, the second (orange) band gave (E) (10) and (Z)-1,2-diferro-cenylethene with a ratio of 3.5:1 (0.47 g, 25%). An analytical sample of 10 was prepared by recrystalliza-tion using the solvent evaporarecrystalliza-tion method from hex-ane:dichloromethane (5:1) at 0°C.
Proton-NMR (CDCl3): d (relative intensity, multi-plicity, assignment): 4.12 (10H, s, Cp2(Fe)); 4.24 (4H, t,
Fig. 1. 2D1H{13C)-HetCOR spectrum of 7 in CDCl 3.
s, – CH). 13C-NMR (CDCl
3): d (assignment): 69.16
(Cp2(Fe)); 66.21 (Cp1(Fe), C(2,5)); 68.54 (Cp1(Fe),
C(3,4)); 84.33 (Cp1(Fe), C(1)); 123.63 ( – CH).
IR(KBr): n (cm− 1) (intensity): 1103 (w), 809 (m), 678
(s).
2.4. Preparation of6inylferrocene (11)
The preparation of vinylferrocene compound 11 was previously reported by Rausch and Seigel [9]. However, a more reliable method is described as following. (1-Hy-droxyethyl)ferrocene (0.52 g, 2.24 mmol), anhydrous cupric sulfate (3 g, 18.73 mmol), and 5 mg of hy-droquinone were dissolved in 100 ml of toluene. The
mixture was refluxed for 45 min and then cooled to room temperature. After the filtration the solvent was removed under vacuum. The residue was extracted with ether and dried with anhydrous magnesium sulfate. The solution was filtered, concentrated to 50 ml under vac-uum. The residue was added to a dry-packed column (1.8 × 9 cm) of silica gel. Elution of the column with hexane gave an orange band which upon removal of the solvent gave vinylferrocene (11) (0.40 g, 84%).
Proton-NMR (CDCl3): d (relative intensity, multi-plicity, assignment): 4.11 (Cp2); 4.21 (2H, t, Cp1
H(3,4)); 4.36 (2H, t, Cp1 H(2,5)); 5.03 (1H, dd, H B,
JAB= 3.3, JBX= 10.8); 5.34 (1H, dd, HA, JAX= 16.8);
Table 3
1H-NMR chemical shifts of selected monosubstituted cynichrodenea, ferroceneband benzenecfrom tetramethylsilane andDd
C6H5–R (C5H5)Fe(C5H4–R) R (CO)2(NO)Cr(C5H4–R) D (ppm) D (ppm) d (ppm) d (ppm) d (ppm) D (ppm) H(4) H(2,5) H(3,4) H(2,5) H(3,4) H(2) H(3)
Electron-withdrawing substituents by resonance
7.44 0.25
–CHO 3 5.77 5.27 0.50 9 4.70 4.47 0.23 7.80 7.55
7.91 7.38 7.48
–C(O)CH3 4 5.72 5.16 0.56 4.66 4.36 0.30 0.43
Electron-donating substituents by resonance 0.00
7.09 6.68 −0.11 –NH2 5 4.6 4.81 −0.21 3.83 3.7 0.13 6.57 0.16 7.35 –CHCH2 6 5.2 5.01 0.19 11 4.36 4.21 0.15 7.51 7.41 7.26 0.26 –CHCH–R% 7 5.25 5.05 0.20 10 4.38 4.24 0.14 7.52 7.36 (R%=(C5H4)Fe(C5H5)) (R%=(C5H4)Cr(CO)2(NO) (E) (R%=C6H5) aFrom [2]. bFrom [13]. cFrom [17].
dD=d[H(2,5)]−d[H(3,4)] for ferrocene and cynichrodene derivatives; d[H(2)]−d[H(4)] for benzene derivatives. The lower-field chemical shift
of each pair is underlined. Table 4
13C-NMR chemical shifts of selected monosubstituted cynichrodenea, ferroceneband benzenecfrom tetramethylsilane andDd
(CO)2(NO)Cr(C5H4-R) R (C5H5)Fe(C5H4-R) C6H5-R D (ppm) d (ppm) d (ppm) D (ppm) d (ppm) D (ppm) C(3) C(4) C(2,5) C(3,4) C(2,5) C(3,4) C(2)
Electron-withdrawing substituents by resonance
129.1 134.0
–CHO 3 93.5 92.8 0.70 96 68.0 72.6 −4.6 129.80 −4.20
128.6 128.5 132.7
–C(O)CH3 4 93.6 92.0 1.6 69.2 71.8 −2.6 −4.10
Electron-donating substituents by resonance 0.00
115.3 129.4 118.7 –NH2 5 73.9 85.1 −11.2 58.8 63.0 −4.2 −3.40 128.3 127.6 –CHCH2 6 88.1 89.7 −1.6 11 66.7 68.6 −1.9 126.1 −1.50 −1.00 127.8 126.8 –CHCH–R% 7 88.6 89.9 −1.3 10 68.5 68.5 −2.3 128.9 (R%=C6H5-R) (R%=(C5H4)Fe(C5H5))
(E) (R%=(C5H4)Cr(CO)2(NO) aFrom [2,18].
bFrom [14]. cFrom [17,19].
dD=d[C(2,5)]−d[C(3,4)] for ferrocene and cynichrodene derivatives; d[C(2)]−d[C(4)] for benzene derivatives. The lower-field chemical shift
of each pair is underlined.
63.53 (Cp1, C(1)); 66.69 (Cp1, C(2,5)); 68.63 (Cp1,
C(3,4)); 69.21 (Cp2(Fe)); 111.02 (CH
2); 134.63
( – CH).
2.5. X-ray diffraction analysis of 7
The intensity data were collected on a CAD-4 dif-fractomer with a graphite monochromator (Mo – Ka radiation). u–2u scan data were collected at room temperature (24°C). The data were corrected for ab-sorption, Lorentz and polarization effects. The absorp-tion correcabsorp-tion is according to the empirical psi rotation. The details of crystal data and intensity collec-tion are summarized in Table 1.
The structure was solved by direct methods and was
refined by full matrix least squares refinement based on
F values. All of the non-hydrogen atoms were refined
with anisotropic thermal parameters. All of the hydro-gen atoms were positioned at calculated coordinates with a fixed isotropic thermal parameter (U = U(at-tached atom) + 0.01 A˚2). Atomic scattering factors and
corrections for anomalous dispersion were from [10]. All calculations were performed on a DEC alpha work-station using the NRCVAX programs [11].
3. Results and discussion
Acid-catalyzed dehydration of alcohol 8 in refluxing benzene, using p-toluene sulfonic acid, on the presence
of hydroquinone as a radical inhibitor, gave olefin 6 in yield as high as 84%. Using anhydrous cupric sulfate instead of p-toluene sulfonic acid, vinylferrocene (11) was obtained from the corresponding alcohol. By reacting with low-valent titanium coupling reagent, prepared from two equivalents of titanium tetrachloride and one equivalent of lithium aluminum chloride, formylcyn-ichrodene (3) was transformed into 7 in 36% yield. An analogous method using titanium trichloride as coupling agent was employed to prepare compound 10 from 9 with 20% yield.
Table 6
Selected bond distances (A˚ ) and angles (°) of 7
Bond distances Cr–C6 2.211(4) Cr–C7 2.206(4) Cr–C9 2.193(4) 2.203(4) Cr–C8 Cr–C10 2.188(4) C6–C7 1.411(6) C6–C10 1.410(6) 1.399(8) C7–C8 1.397(7) C8–C9 1.750(4) Cr–N1 1.393(7) C9–C10 1.812(4) Cr–C2 Cr–C3 1.823(4) N1–O1 1.161(4) C2–O2 1.154(5) 1.144(5) C3–O3 C5–C6 1.471(6) C5–C5 1.288(8) 1.849 Cr···centroid(Cp) Cr···C5 3.287 3.116 H(C5)···H(C10) 3.79 C5···C10 Bond angles C6–C7–C8 109.0(4) C7–C8–C9 107.8(4) 107.9(4) C7–C6–C10 106.1(4) C8–C9–C10 109.1(4) CO–C10–C9 N1–Cr–C2 93.10(17) 93.30(17) N1–Cr–C3 C2–Cr–C3 93.26(18) 179.8(3) Cr–N1–O1 Cr–C2–O2 178.4(4) Cr–C3–O3 179.1(4) C5–C6–C7 124.4(4) 129.4(4) C5–C6–C10 C5–C5–C6 125.5(4) Centroid(Cp)–Cr – N1 126.8 120.4 centroid(Cp)–Cr–C3 121.6 Centroid(Cp)–Cr–C2
Compound 7 exhibits two carbonyl stretching bands, the symmetric mode occurring at 2010 cm− 1 and the
asymmetric mode at 1932 cm− 1. The nitrosyl stretching
band is observed at 1682 cm− 1. It is interesting to
compare the three stretching frequencies of 7 with the corresponding bands of its unsubstituted parent com-pound 1 (2025, 1955; 1695 cm− 1). The lower-frequency
shift by 15, 23 and 13 wave-numbers indicate that the vinyl group is exerting as an electron donating group to the two adjacent cynichrodene moieties. The data is consistent with the result found from the study of the
13C-NMR spectra.
Table 5
Atomic parameters x, y, z and Beqaof 7, estimated S.D.s refer to the
last digit printed
Beq z y x 0.07397(5) 0.22020(5) 3.40(3) 0.25739(9) Cr 0.4863(5) 0.2069(3) 0.1757(3) 4.73(17) N1 0.3740(6) 0.1406(4) C2 −0.0326(3) 3.88(19) 0.1365(6) 0.0874(4) C3 0.1225(3) 3.86(19) 0.4368(7) 0.4630(3) C5 −0.0293(4) 4.13(22) 0.2558(6) 0.4036(3) C6 0.0092(3) 3.39(18) 0.1003(7) 0.3375(4) C7 −0.0618(4) 4.28(21) − 0.0516(7) 0.2978(4) C8 0.0019(5) 5.01(25) 0.0069(7) 0.3390(4) C9 0.1130(5) 4.83(24) 0.4036(4) 4.10(21) 0.1941(7) 0.1175(4) C10 6.82(18) 0.1433(3) 0.1979(3) 0.6377(5) O1 0.4499(5) 0.0924(3) O2 −0.10142(25) 6.69(18) O3 0.0628(5) 0.0039(3) 0.1540(3) 6.92(19) 0.519(6) 0.546(3) H5 0.104(3) 6.1(11) 0.097(5) 0.325(3) −0.142(3) H7 3.2(8) −0.177(6) 0.252(4) H8 −0.024(3) 7.0(12) 4.2(9) H9 −0.057(5) 0.332(3) 0.180(3) 2.7(9) H10 0.260(5) 0.437(3) 0.172(3) aB
Fig. 3. View of 7 along the normal of Cp(Cr) ring.
The1H-NMR spectra of 6, 7, and 10 were consistent
with their structures and similar to other metallocenyl systems [12,13]. The strong diamagnetic anisotropic effect of the vinyl group on the ring protons might explain why the protons (2- and 5-positions) closer to it were deshielded to a greater extent than those (3- and 4-positions) further away from it.
From Table 2, the contracted 2D HetCOR spectra of
2 – 7 (Fig. 1), it is interesting to find that compounds 2 – 4 exhibit positive slopes, while 6 and 7 exhibit
nega-tive slopes. The opposite slopes may be interpreted as follows. First, the strong diamagnetic anisotropic ef-fects of the carbonyl and vinyl group on the ring protons might explain why the protons (2- and 5-posi-tions) closer to it were deshielded to the lower field for all compounds of 2 – 4 and 6 – 7. These results are in agreement with the corresponding benzene and fer-rocene analogues (Table 3). Second, the shielding of C(2,5) and C(3,4) carbons atoms of 6 and 7 are analogous to 5 instead of 2 – 4 and suggests that the electron density distribution in the cyclopentadienyl rings of 6 – 7 is similar to 5, rather than 2 – 4.
The assignment of13C-NMR spectra of 6, 7, and 10
were based on standard13C-NMR correlations [14], 2D
HetCOR(Table 2), the DEPT technique and by com-parison with other metallo-aromatic systems [15].
It is interesting to compare the 13C-NMR spectra of
3, 6, and 7 with their unsubstituted parent compound 1
(Table 4). For the carbon atoms on Cp(Cr) (C(3,4) and C(2,5)), the chemical shifts of 3 occur at a lower field than the chemical shifts of 1 at d=90.31 ppm. How-ever, the chemical shifts of 6 and 7 occur at a higher field than d=90.31 ppm. This reflects the strong elec-tron-withdrawing effect of the formyl group on 3 and the strong electron-donating effect of vinyl group on 6 and 7. It is worth pointing out from Table 4 that the chemical shifts of C(3,4) occur at a higher field than the chemical shifts of C(2,5) for 3 and 4. On the contrary,
the chemical shifts of C(3,4) occur at a lower field than the chemical shifts of C(2,5) for 5 – 7. Earlier, we have reported thorough spectra studies on aminocynichro-dene (5) [2], a compound with a strong electron-donat-ing substituent, – NH2 on Cp. We also compared the chemical shifts of selected monosubstituted cynichro-dene derivatives with the NMR data of their analogues of ferrocene and benzene derivatives (Table 4). The large contribution of canonical form 5a to 5 explained the relatively large negative difference D (−11.2) in C(2,5) and C(3,4). This is understandable in the stabi-lization of chromium anion because of the overall electron-withdrawing properties of CO and NO ligands.
As is well known, the vinyl group can exert an either electron-donating or electron-withdrawing effect by res-onance to its attached group. When an electron-donat-ing group is attached to it, the vinyl group withdraws the electron from the group, and vice versa.
Upon examination of the infrared spectra of 6 and 7 (Tables 2 and 4), the following conclusions may be drawn:
1. the vinyl group exerts the same kind of electronic effect as the amino group, rather than carbonyl group, to its adjacent Cp ring;
2. in compounds 6 and 7, the vinyl group donates electrons to the adjacent Cp ring(s), which in turn transfers the electron density to the metal chromium. This resulted in an enhanced p-back bonding phenomena observed in IR spectra; 3. canonical 7a, rather than 7b, to some extent
Table 7
Selected structural data of 7, 12 and 13a
Compound Bond length (A˚ )
Cr–C–O vCrb(°) uCrc(°)
Cr–C(ring) Cr–NO Cr–CO NO CO Cr–N–O Cr···exocyclic
atom 3.287 −1.2 76 2.200(4) 1.750(4) 1.812(4) 1.161(4) 1.154(5) 179.8(3) 178.4(4) 46.5 179.1(4) 1.823(4) 1.144(5) −2.0 8.3 3.275 12 2.195(22) 1.747(18) 1.798(18) 1.187(23) 1.126(27) 174.5(16) 177.0(18) 177.2(19) 1.828(23) 1.160(22) 3.258 −2.1 9.7 2.177(25) 1.807(33) 1.745(21) 1.189(40) 1.130(20) 179.2(27) 173.8(25) 1.758(17) 1.149(25) 179.8(25) 1.0 177.2 3.223 13 2.205(5) 1.712(4) 1.864(4) 1.178(5) 1.135(5) 179.4(3) 179.0(4) 177.2(4) 1.846 1.135(5) a12 (CO) 2(NO)Cr[(h 5-C 5H4)–NH–C(O)–NH–(h 5-C 5H4)]Fe(h 5-C 5H5); 13 (CO)2(NO)Cr(h 5-C 5H4)C(O)(h 5-C 5H4)Fe(h 5-C 5H5). bv
Cr(°): the twist angle is defined as the torsional angle between the nitrosyl nitrogen atom, the chromium atom, the Cp center and the ring
carbon atom bearing the exocyclic carbon atom.
cu
Cr(°): the angleu is defined as the angle between the exocyclic bond and the corresponding Cp ring with positive angle towards metal and
negative angles away from the metal.
The molecular structure of 7 is shown in Fig. 2. The atomic coordinates of the non-hydrogen atoms are listed in Table 5. Selected bond distances and angles are given in Table 6.
Compound 7 adopts a transoid conformation at the organic vinyl carbons. The coordination geometry about the Cr center is approximately a distorted tetra-hedron with two carbonyl groups, the Cp group and nitrosyl group as the four coordination sites. The nitro-syl group is located at the side toward the exocyclic carbon atom of Cp(Cr) with a twist angle of 46.5° (Fig. 3). The twist angle is defined as the torsional angle between the nitrosyl nitrogen atom, the chromium atom, the Cp center and the ring carbon atom bearing the exocyclic carbon atom.
In the cynicrodene moiety, the observed average bond length of Cr – C(ring) is 2.200(4) A˚ . The Cr–N length of 1.750(4)(Cr – N1) is closer to those found in (CO)2(NO)Cr[(h5-C5H4) – NH – C(O) – NH – (h5-C5H4)]
Fe(h5-C
5H5) (12) [2](1.747(18), 1.803(33) A˚ ) than the
values found in (CO)2(NO)Cr(h5-C5H4)C(O)(h5
-C5H4)Fe(h5-C5H5) (13)(1.712(4) A˚ ) [1]. The Cr–C
(car-bonyl) distances of 1.8l2(4)A˚ (Cr–C2) and 1.823(4) A˚ (Cr – C3) are between those found in 12 (average 1.782(23) A˚ ) [2] and those found in 13 (average 1.855(4) A˚ ) [1]. The Cr–N–O angle of 179.8(3)° is consistent with the NO+ formalism typical of the linear M – NO
linkage. The Cr – C – O angles of 178.4(4)° (Cr – C2 – O2) and 179.1(4)° (Cr – C3 – O3) indicate the usual mode of bonding in the terminal metal carbonyl complexes. The Cr – centroid (Cp(Cr)) distance of 1.849 A˚ agrees with the values of 1.843 A˚ in 12 [2] and 1.846 A˚ in 13 [1]. The average C – C distance in the ring (Cp(Cr)) is 1.402 A˚ . Selected structural data of 7, 12 and 13 are listed in Table 7.
The exocyclic CC bond length is 1.228(8) A˚ (C5– C5). The exocyclic carbon atom C5 is bent away from the corresponding Cr atom, with an angle u= −1.2°. The angle u is defined as the angle between the exo-cyclic C6 – C5 bond and the corresponding Cp ring with positive angle towards metal and negative angle away from the metal. The vinyl plane turns away from the ring planes by 9.1°. The two Cp planes are coplanar. Upon examination of the bonds of Cr – NO, Cr – CO, NO, and CO, it is interesting to find that 7 has the longer bond distances of Cr – NO and CO, while it has the shorter bond distances of Cr – CO and NO than 13 (Table 5). The contributions of canonical form 7ii to 7 and 13i to 13 may explain such results.
Previously, McGlinchey [16] made EHMO calculations on compound [(CO)2(NO)Cr(C5H4– CH2)]+ (14).The
symmetric isomer 14i is favored by 2.7 kcal mol− 1over
the unsymmetrical rotamer 14ii.
The calculation is consistent with the X-ray structure of 13, a compound with its Cp rings bearing an elec-tron-withdrawing group. The twist angle of 13 is 177.2° (Fig. 5, Table 7). The preference for the symmetrical isomer of 13i is related to the ability of the exocyclic double bond to donate electron density to the chromium atom such that it is trans to the better p-accepting ligand, i.e. NO+. As a result, the shorter
bond distance of Cr – NO and the longer bond distance of NO are observed. Conversely, for compound 7, its Cp rings bearing an electron-donating group, the un-symmetrical rotamer 7ii having a longer Cr – NO and shorter NO bond distances is preferred.
The X-ray structural data also support the EHMO calculation. In 7 a negative u value (−1.2°) and a longer bond distance between chromium atom and exocyclic carbon (3.287 A˚ ) was obtained; while a posi-tive u value (1.04°) and a shorter bond distance be-tween chromium atom and exocyclic carbon (3.223 A˚ ) was obtained in 13. The molecular structures of 12 and
13 are shown in Figs. 4 and 5, respectively.
4. Supplementary material available
A list of anisotropic temperature factors of non-hy-drogen atoms and the coordinates with isotropic tem-perature factors of hydrogen atoms as well as list of structure amplitudes (6 pp.) have been deposited. Or-dering information can be obtained from the authors.
Acknowledgements
The authors are grateful to the National Science Council of Taiwan for grants in support of this research program.
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